1. Field of the Invention
The present invention relates to a process for the continuous preparation of acyloxysilanes by reacting organochlorosilanes with essentially an excess of monocarboxylic anhydride at elevated temperature, separating the acyl chloride formed as by-product in the reaction and working up the remaining reaction mixture.
2. Description of the Background
Acyloxysilanes have many applications in the chemical industry. They are suitable, for example, as cross-linking silicon compounds in the preparation of compositions which are storable in the absence of water and hardenable at room temperature on exposure to moisture. Examples of such acyloxysilanes include methyl-, ethyl- and propyltris(ethanoyloxy) silane.
For silanes of this type, recently, there have been extreme increases in the requirements relating to their use with regard to purity, in particular with respect to the residual content of chlorine in the form of acid chloride. Residual chlorine contents in acyloxysilanes occur because of the incomplete reaction of all of the chlorine constituents of the organochlorosilane starting materials with the reagents which introduce the acyloxy group into the reaction. In the present invention, acid chloride is taken to mean chlorine which is bound to the silicon atom in a silane, hydrogen chloride, and chlorine present in an acyl chloride.
To prepare acyloxysilanes on an industrial scale, essentially two procedures are employed.
One procedure makes use of the reaction of organochlorosilanes with carboxylic acids to give the corresponding organoacyloxysilanes. This procedure is performed both batchwise and continuously. Batchwise preparation requires the presence of an inert solvent and long reaction times, in order to lower the residual acid chloride contents to values below 100 ppm (U.S. Pat. Nos. 2,437,073, 2,866,800, 3,974,198, GB814 011). Continuous reactions of organochlorosilanes with carboxylic acids are disclosed in the publications DE 2 801 780, DE 3 221 702, EP 0 003 317, U.S. Pat. Nos. 4,332,956, 4,329,484. When use is made of small excesses of carboxylic acid, based on the amounts of organochlorosilane used, acid chloride contents remain of the orders of magnitude of up to 50 ppm by weight. Only the application of great excesses of carboxylic acid of the order of magnitude of 50% enables the acid chloride contents to be decreased to values below 1 ppm by weight, with unsatisfactory siloxane contents being tolerated.
The other procedure for preparing acyloxysilanes on an industrial scale makes use of the reaction of organochlorosilane with carboxylic anhydrides to give the corresponding organoacyloxysilanes.
FR 1 003 073 describes the batchwise and simultaneous preparation of acyloxysilanes and acyl chlorides by reading organochlorosilanes with monocarboxylic anhydrides. However, unsatisfactory product yields are obtained with undetectable residual chlorine contents being obtained after a reaction time of 8 hours.
A continuous method of preparing acyloxysilanes is disclosed in EP 0 509 213, in which organochlorosilanes are reacted with carboxylic anhydrides in the presence of special catalysts to give acyloxysilanes and acyl chlorides. The special procedure enables the acid chloride content to be decreased in the end products from, for example, 800 ppm by weight to 3 ppm by weight. However, lowering the acid chloride contents below the limits of detection can only be achieved with this procedure at the cost of greatly increased reaction times with the use of increased amounts of carboxylic anhydride.
The literature also describes procedures for preparing acyloxysilanes by reacting corresponding organochlorosilanes with alkali metal carboxylates in the presence of large amounts of inert diluents. These procedures are associated with the production of considerable amounts of salt and the need to remove the target product from the reaction mixture by complex washing processes. These procedures are scarcely practical on an industrial scale and also do not comply with current economic and ecological requirements (U.S. Pat. No. 2,573,302, GB 640 834, DE 870 554, U.S. Pat. Nos. 2,537,073, 2,866,800).